1. Field of the Invention
The invention relates to cubic crystalline optical elements, particularly calcium fluoride, in forms such as prisms for propagating highly collimated, linearly polarized light along predetermined crystal axes and to the manufacture and use of such cubic crystalline optical elements.
2. Description of Related Art
The short-wavelength high-photon-energy content of deep ultraviolet (UV) light (wavelengths between approximately 120 nanometers and 250 nanometers) significantly limits the number of materials capable of transmitting ultraviolet light and amplifies weaknesses in the limited number of materials that do transmit such ultraviolet light. Most ordinary optical materials are either insufficiently transmissive or subject to breakdown by absorption.
Calcium fluoride (CaF2) is among the optical materials capable of transmitting deep UV light. Despite calcium fluoride's highly symmetric cubic crystalline structure, calcium fluoride exhibits intrinsic birefringence at the short wavelengths. Different polarization components of the ultraviolet light experience different refractive indices depending upon both the direction of polarization and the direction of propagation of the light through the crystal material. A description of the nature of the intrinsic birefringence in calcium fluoride crystals is found in a paper by John Burnett, Zachary Levine, Eric Shirley, and John Bruning entitled “Symmetry of spatial-dispersion-induced birefringence and its implications for CaF2 ultraviolet optics”, Society of Photo-Optical Instrumentation Engineers, J. Microlith., Microfab., Microsyst. p. 213-224, October 2002, which is hereby incorporated by reference.
There are 14 crystal axes, which can be referenced according to Miller indicies, along which deep ultraviolet light can be propagated through calcium fluoride crystals without encountering intrinsic birefringence effects. These include propagations in both directions along the three cube axes (i.e., the family of <1 0 0> directions) and along the four cube body diagonals (i.e., the family of <1 1 1> directions, referred to herein as the <1 1 1> main axis). The <1 1 1> main axis is a normal to the natural {1 1 1} cleavage plane of calcium fluoride crystals, referred to herein as the {1 1 1} primary plane. The {1 1 1} primary plane is the plane along which the crystal tends to break first when struck. Optical elements made from calcium fluoride crystals are generally oriented for propagation of light along the <1 1 1> main axis, because the natural cleavage of calcium fluoride crystals presents planar surfaces normal to the crystal's <1 1 1> main axis.
Prisms for use in the deep UV are frequently cut out of calcium fluoride crystals grown in rod form with either ordered or randomly oriented crystal axes. One end of the rods is cleaved along the {1 1 1} primary plane (normal to the <1 1 1> main axis) to provide a basis for prism orientation. The sides of the oriented rods are cut and polished to form slabs. A line of prisms are traced on the slab, and the slab is further cleaved for orienting each of the prisms to support the propagation of light along the <1 1 1> main axis of the crystal.
The crystal rods are expensive to manufacture, considerable waste is associated with the formation of the slabs, and a separate cleave is generally required for each prism. A higher yield from these rods would be desirable.